Optical lens driven by electrowetting and of variable focal length are described in European Patent EP-B1-1,166,157, the content of which is hereby incorporated by reference. A cell is defined by two transparent insulating plates and side walls. The lower plate, which is non-planar, comprises a conical or cylindrical depression or recess, which contains a drop of a non-conductive or insulating liquid. The remainder of the cell is filled with an electrically conductive liquid, non-miscible with the insulating liquid, having a different refractive index and substantially the same density. An annular electrode, which is open facing the recess, is positioned on the rear face of the lower plate. Another electrode is in contact with the conductive liquid. Through electrowetting phenomena it is possible to modify the curvature of the interface between the two liquids, according to the voltage V applied between the electrodes. Thus, a beam of light passing through the cell normal to the plates in the region of the drop will be focused to a greater or lesser extent according to the voltage applied. The conductive liquid generally is an aqueous liquid containing salts. The non-conductive liquid is typically an oil, an alkane or a mixture of alkanes, possibly halogenated.
In order to achieve a very performing optical lens, i.e. an optical lens being reliable and having a good optical quality measured as a low wave front error, a low response time and similar performances over a broad temperature range, the conductive liquid and the non-conductive liquid should meet a lot of specific requirements such as for example being immiscible, having substantially the same density, remain liquid in a broad temperature range, being chemically stable when in contact with each others, being as compatible as possible with the insulating plates and side walls encasing said electrowetting device, and having a predetermined refractive index difference.
The number of chemical compounds or mixture of compounds that can be used in the liquids and reaching all the above cited requirements is very limited.
It is known in the art that salts can be used as anti-freezing agents in the conductive liquid of optical electrowetting devices so that they can be operational and stored under 0° C. temperatures. Traditionally such devices, especially liquid lenses, have to be operational down to −10° C. and can be stored at temperature below −20° C. U.S. Pat. No. 7,242,528 discloses for example salts such as LiCl used in the conductive liquid of an electrowetting module to decrease the freezing point to below −20° C. US 20070179201 in the name of the Applicant describes the use of bromine anion and other freezing-point lowering agents in conductive liquid. US 20070091455 discloses an electrowetting system wherein the conductive liquid contains a mixture of salts to decrease the freezing point while minimizing changes in some physical properties of the conductive liquid.
A drawback with known prior art is an excessive increase of the refractive index of the conductive liquid when comprising salts as anti-freezing agents. Since the conductive liquid has preferably a refractive index lower than the refractive index of the non-conductive liquid, the use of salts as freezing point lowering agents according to the prior art tends to decrease the difference in refractive index between the conductive and the non-conductive liquids, which is undesirable in many applications, such as for example in zoom applications.
In these applications it is desirable to provide optical electrowetting devices showing high refractive index difference between the two liquids forming the optical interface over a broad temperature range, while filling all the previous cited requirements to ensure a good performance of the optical electrowetting devices.
Another drawback with known prior art on optical electrowetting device such as optical lenses or zooms is the degradation of the optical performance of the device due to chromatic aberrations that can lead to blurred images. In particular, increasing the amount of salts can lead to an increase of the chromatic aberrations. Chromatic aberrations are due to the variation of the refractive index of a material forming an optical element according to the wavelengths of light, also known as the dispersion of the lens. In the case of a lens, chromatic aberrations are the failure of a lens to focus all colors to the same point. Chromatic aberrations can be a problem difficult to overcome in optical design, especially when using for example an optical electrowetting device wherein the optical power is changing according to the application of a voltage, thus providing variable chromatic aberrations. This problem can be even greater for zoom applications wherein a large optical power variation is required, for example a zoom where the zoom function is provided by an optical electrowetting device having a large optical power variation, for example a device with the two liquids forming the optical interface by having a high refractive index difference. It is thus also desirable to provide optical electrowetting devices showing low chromatic aberrations.